Facial Psoriasis Log‐based Area and Severity Index: A valid and reliable severity measurement method detecting improvement of facial psoriasis in clinical practice settings

Facial psoriasis is often observed in moderate to severe degrees of psoriasis. While we previously demonstrated construct validity of the facial Psoriasis Log‐based Area and Severity Index (fPLASI) system for the cross‐sectional evaluation of facial psoriasis, its reliability and accuracy to detect clinical improvement has not been confirmed yet. The aim of this study is to analyze whether the fPLASI properly represents the range of improvement for facial psoriasis compared with the existing facial Psoriasis Area and Severity Index (fPASI) after receiving systemic treatments in clinical practice settings. The changing severity of facial psoriasis for 118 patients was calculated by the scales of fPASI and fPLASI between two time points after systemic treatments. Then, percentage changes (ΔfPASI and ΔfPLASI) were analyzed from the perspective of both the Physician's Global Assessment of effectiveness (PGA) and patients’ Subjective Global Assessment (SGA). As a result, the distribution of the fPASI was more heavily clustered around the low score range compared with the fPLASI at both first and second visits. Linear regression analysis between ΔfPASI and ΔfPLASI shows that the correlation coefficient was 0.94, and ΔfPLASI represented greater percentage changes than ΔfPASI. Remarkably, degrees of clinical improvement measured by the PGA matched better with ΔfPLASI, while ΔfPASI underestimated clinical improvements compared with ΔfPLASI from treatment‐responding groups by the PGA and SGA. In conclusion, the fPLASI represented clinical improvement of facial psoriasis with more sensitivity and reliability compared with the fPASI. Therefore, the PLASI system would be a viable severity measurement method for facial psoriasis in clinical practice.